1. Field of the Invention
This invention relates to a light quantity adjusting device for a camera, a projector or the like.
2. Description of the Related Art
A conventional light quantity adjusting device is described with reference to FIGS. 24(a) and 24(b) as follows: A rotor 113 made of a magnet is magnetized to have two poles and is encompassed with two bobbins 114a and 114b, which are obtained by molding and machining a plastic material. Copper wires are wound around the two bobbins 114a and 114b to form coils 115a and 115b. The bobbins 114a and 114b which are thus provided with the coils 115a and 115b are encompassed with a yoke 119, which is made of soft iron into a ring shape. The rotor 113 is thus arranged to rotate under the influence of a magnetic flux generated when a DC current flows to the coils 115a and 115b. The rotation of the rotor 113 is transmitted through an arm 116 to diaphragm blades 117a and 117b of a diaphragm. The diaphragm blades 117a and 117b then move in the direction of opening the aperture of the diaphragm. A coiled spring (hereinafter referred to as a return spring) 118 is attached to the arm 116 and is arranged to constantly exert a force on the diaphragm blades 117a and 117b in the direction of closing the aperture which is formed jointly by them. A magnetism detecting element 128 such as a Hall element is employed as an angular position detector for detecting the angular position of the rotor 113. The diameter of the aperture formed by the diaphragm blades 117a and 117b is arranged to be enlarged or reduced by electrically adjusting a turning force which is caused by a magnetic action and exerted in the opening direction. An electrical circuit arrangement provided for this purpose is omitted from the description given here.
In the case of the conventional device, the whole outer circumferential part of the rotor 113 is covered with the bobbins 114a and 114b which have the copper wires wound around them. Therefore, as shown in FIG. 24(b), an air gap between the rotor 113 and the ring-shaped yoke 119 inevitably becomes too large. Hence, in order to obtain a driving torque required for light quantity adjustment, it has been necessary either to increase the diameter of the rotor 113 or to use a magnet having a large maximum energy product. This has caused an increase in size of a driving part or necessitated use of an expensive magnet. Further, since the coils 115a and 115b must be located always close to the rotor 113, it has been theoretically impossible to shift their locations. Therefore, it has been hardly possible to increase the efficiency of space utilization. This not only has caused an increase in size of the driving part but also allowed a less amount of latitude of layout to hinder reduction in size of a lens barrel, with respect to matching with the lens barrel. Further, the use of the coiled spring 118 for providing a force required in closing the diaphragm blades 117a and 117b has lowered work efficiency because of difficulty in mounting it and also has greatly hindered the uniformalization in quality of products. Another problem with the conventional device lies in a mechanical friction caused by the coiled spring 118 when the diaphragm blades 117a and 117b make sliding movement. The mechanical friction tends to prevent adequate braking and cause vibrations of the diaphragm blades 117a and 117b.
Further, the terminals of the magnetism detecting element 128 and those of the coils 115a and 115b are arranged not on one and the same plane and also away from each other at a relatively long distance. This arrangement results in a complex shape of a flexible circuit board provided for interconnecting these terminals and thus has caused a poor assembly workability and an increase in cost. A further problem of the conventional device lies in lowered workability caused by a method of fixing the magnetism detecting element 128 by taping.
A method for constantly urging the rotor in one direction by utilizing a cogging torque has been disclosed in Japanese Utility Model Application Laid-Open No. HEI 3-29837. According to the method disclosed, the rotor is urged in one direction by the cogging torque when the coil is not energized and in the other direction by a composite torque obtained from an energization torque and the cogging torque when the coil is energized.
Further, the driving part of a light quantity adjusting device according to this invention completely differs in uses from such stepping motors that are described in Japanese Patent Application Laid-Open No. SHO 57-68661, Japanese Patent Application Laid-Open No. SHO 55-133663, etc. The driving part of this invention is uniquely arranged, particularly with respect to driving control over the diaphragm of a camera.